Conventionally, there is known an SDMA (Spatial Division Multiple Access) scheme which spatially multiplexes a plurality of users using a plurality of transmission antennas on the same frequency band at the same time. If the SDMA scheme is applied to a wireless communication system in which a base station and a plurality of user terminals (wireless communication apparatuses capable of, at least, receiving a wireless signal) communicate with each other, communication is possible without spatial interference between the users.
As an SDMA scheme, various embodied schemes have been proposed. In a ZF (Zero-Forcing) scheme, for example, communication is performed without interference between users by generating a matrix (to be referred to as a channel matrix hereinafter) having, as its elements, channel coefficients indicating propagation channel states between a plurality of transmission antennas of a base station and the reception antennas of a plurality of user terminals, and multiplying a transmission signal (user signal) by the pseudo-inverse matrix of the channel matrix as a weight. If the above weight multiplication process is performed when the spatial correlation for the channel matrix is high, the signal level of the transmission signal increases. In the ZF scheme, therefore, the transmission signal is additionally multiplied by a normalization coefficient so that its transmission power does not exceed a rated transmission power. In the ZF scheme, since the above normalization coefficient multiplication process causes a power loss of the transmission signal, noise enhancement occurs upon performing channel equalization for a reception signal in a wireless communication apparatus on the reception side, thereby deteriorating a reception performance. Note that noise enhancement becomes larger as the inverse number of the normalization coefficient increases.
In a VP (Vector Perturbation) scheme described in B. Hochwald, C. Peel, A. Swindlehurst, “A Vector-Perturbation Technique for Near-Capacity Multiantenna Multiuser Communication—PartII: Perturbation,” IEEE Trans. on Communications, Vol. 53, No. 3, pp. 537-544, March 2005 (hereinafter referred to as the “reference 1”) and C. Windpassinger, R. Fischer, and J. Huber, “Lattice-Reduction Aided Broadcast Precoding,” IEEE Trans. on Communications, Vol. 52, No. 12, pp. 2057-2060, December 2004 (hereinafter referred to as the “reference 2”), a so-called perturbation vector which can extend the signal point of a transmission signal is used. The VP scheme searches for a perturbation vector which shifts a transmission signal to an extended signal point such that the inverse number of the normalization coefficient is minimized, adds the searched perturbation vector to the transmission signal, and performs weight multiplication and normalization coefficient multiplication. A wireless communication apparatus on the reception side can reconstruct the transmission signal before the perturbation vector is added, by removing the perturbation vector from a received signal using a modulo operation. Even in the VP scheme, noise enhancement occurs like the ZF scheme. Since, however, the inverse number of the normalization coefficient is small as compared with the ZF scheme, it is possible to suppress deterioration of a reception performance.
Conventionally, a transmission signal in a wireless communication system contains a pilot signal for channel estimation in addition to a data signal as a substantial reception target. The same transmission scheme is generally applied to the data signal and pilot signal. For example, if the data signal is multiplied by a weight, a received data signal has been multiplied by the weight in addition to the channel coefficient of a propagation channel. This requires a wireless communication apparatus on the reception side to estimate not only the channel coefficient but also an effective channel considering the weight. A wireless communication apparatus on the transmission side, therefore, needs to multiply the pilot signal by the weight so that the wireless communication apparatus on the reception side can estimate the effective channel.
In terms of a reception performance, it is not always preferable to simply apply the same transmission scheme to the data signal and pilot signal in wireless communication using a perturbation vector like the above-mentioned VP scheme. As described above, since the wireless communication apparatus on the reception side uses the pilot signal for channel estimation, the pilot signal has a value known to the wireless communication apparatus. Since, however, a perturbation vector has a value unknown to the wireless communication apparatus on the reception side, the wireless communication apparatus cannot estimate a correct effective channel if the perturbation vector is added to the pilot signal. Furthermore, since the value of a searched perturbation vector varies depending on an addition target signal, perturbation vectors which are respectively added to the data signal and pilot signal are not always the same. Therefore, normalization coefficients which are respectively calculated for the data signal and pilot signal are not always the same.
References such as references 1 and 2 are based on the premise that parameters such as a perturbation vector (which is added to the data signal) and a normalization coefficient (by which the data signal is multiplied), which are normally unknown to the wireless communication apparatus on the reception side are known, and the wireless communication apparatus can perform ideal channel equalization. That is, the above references do not disclose a particular technique for enabling to actually perform wireless communication using a perturbation vector, for example, a practical estimation technique for an effective channel.